The present invention relates generally to ferroelectric ceramics and, in an embodiment described herein, more particularly provides a ferroelectric ceramic having enhanced usefulness in a pyroelectric infra-red detector.
The use of ferroelectric ceramics as the active detector material for pyroelectric infra-red (IR) detectors is known. The range of pyroelectric devices and the various single crystal, polymeric and ceramic materials which have been used as the active material in them is described in detail in, for example, a paper entitled "Pyroelectric Devices and Materials" by R. W. Whatmore, 49 Rep. Prog. Phys. 1335-1386 (1986), which is incorporated herein by this reference. The paper also discloses figures-of-merit which can be used to decide whether or not one material is more suitable than another for a particular type of pyroelectric device. These figures-of-merit are various combinations of the physical properties of the pyroelectric material and are directly related to the performance of the devices of interest. The most commonly used figures-of-merit are: EQU F.sub.i =p/c' (1)
where the current responsivity of the device is proportional to F.sub.I ; EQU F.sub.v =p/(c'.di-elect cons..sub.0.di-elect cons.) (2)
where the voltage responsivity of the device is proportional to F.sub.v ; and EQU F.sub.D =p/(c'(.di-elect cons..sub.0.di-elect cons. tan .delta.).sup.0.5) (3)
where the specific detectivity of the device is proportional to F.sub.D.
In these formulae:
p=pyroelectric coefficient; PA1 c'=volume specific heat; PA1 .di-elect cons.=dielectric permittivity at the frequency of device operation; PA1 tan .delta.=dielectric loss tangent at the frequency of device operation; and PA1 .di-elect cons..sub.0 =dielectric permittivity of free space. PA1 0.4.gtoreq.x.gtoreq.0 PA1 0.4.gtoreq.y&gt;0 PA1 0.05.gtoreq.z&gt;0 PA1 0.25.gtoreq.x&gt;0 PA1 0.24.gtoreq.y&gt;0 PA1 0.88.gtoreq.(1-x) (1-y) PA1 0.05.gtoreq.z&gt;0 PA1 x=0.125.+-.0.01, y=0.025.+-.0.01, z=0.01.+-.0.002, A=Mn; PA1 x=0.075.+-.0.01, y=0.075.+-.0.01, z=0.01.+-.0.002, A=Mn; PA1 x=0.175.+-.0.005, y=0.025.+-.0.005, z=0.0065.+-.0.001 and A=U.
In an ideally-matched pyroelectric device the input capacitance of the amplifier linked to the pyroelectric element would be similar in magnitude to that of the element itself. In this cases the figure-of-merit F.sub.D is the most important one to use. In devices where the element capacitance is much larger than the amplifier capacitance, or where the AC Johnson Noise in the element does not dominate in the noise figure for the device, F.sub.V is the appropriate figure-of-merit to use. In cases where the element capacitance is much smaller than the amplifier capacitance, then F.sub.i is the appropriate figure-of-merit to use.
Table 1 shows the pyroelectric properties of some of the commercially-available materials for pyroelectric applications. All the commercial pyroelectric ceramic materials are based upon modifications to the perovskite ceramic solid solution system lead zirconate--lead titanate (PbZrO.sub.3 --PbTiO.sub.3 --hereinafter called "PZT"). The vast majority of these are based upon lead titanate (PbTiO.sub.3 --hereinafter called "PT"). An example is the composition (Pb.sub.1-x Ca.sub.x)((Co.sub.1/2 W.sub.1/2)yTi.sub.1-y)O.sub.3 with x=0.24, y=0.04, which was described in a paper by N. Ichinose, 64 Am. Ceram. Soc. Bull. 1581-1585 (1985). Typical properties for such ceramics are listed under the heading of "modified PT" given in Table 1. These values have been measured on a modified lead titanate ceramic manufactured by Morgan Matroc Unilator Division and known as PC6.
A particular commercial family of pyroelectric ceramics has also been developed based upon PZT compositions close to lead zirconate, (PbZrO.sub.3 --hereinafter called "PZ") in this case in solid solution with lead iron niobate (PbFe.sub.0.5 Nb.sub.0.5 O.sub.3) which was described in a paper by R. W. Vtiatmore and A. J. Bell (35 Ferroelectrics 155-160 (1981)). In this case uranium is added as a dopant to control the electrical conductivity. The properties of this ceramic, which is supplied commercially by GEC Marconi Materials Techluology are listed in Table 1 under heading "Modified PZ".
Other authors have described ceramic compositions which are particularly suitable for piezoelectric applications. A paper by H. Ouchi, K. Nagano and S. Hayakawa (48 J. Amer. Ceram. Soc. (12) 630-635 (1965)) discloses the piezoelectric and high frequency (&gt;1 KHz) dielectric properties of coinpositions throughout the phase diagram. The compositions disclosed in this reference are based on PZ, PT and lead magnesium niobate (hereinafter called "PMN"). It should be noted that none of the properties reported in this reference would be of use in predicting the properties of a pyroelectric device using them. While the dielectric properties (permittivity and loss) might at first sight seem useful in providing some data for the computation of pyroelectric figures-of-merit, according to the formulae given here, the frequency at which the properties are measured should be in the same range as those used in practical pyroelectric devices (usually &lt;100 Hz). This is particularly important for the dielectric loss, which can rise rapidly as the frequency is reduced below 100 Hz. Compositions cited in this reference are shown in the PZ-PT-PMN ternary phase diagram shown in FIG. 1.
A paper by H. Ouchi, M. Nishida and S. Hayakawa (49 J. Amer. Ceram. Soc. (11) 557-582(1966)) discloses the piezoelectric and high frequency (&gt;1 KHz) dielectric properties of a much more restricted set of compositions of the form Pb[(Mg.sub.1/3 Nb.sub.2/3).sub.0.375 Zr.sub.0.375 Ti.sub.0.25 ]O.sub.3 with small additions (0.2 to 1.0 m Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3 or NiO.
A paper by H. Ouchi (51 J. Amer. Ceram. Soc. (3) 169-176 (1968)) discloses the piezoelectric and high frequency (&gt;1 KHz) dielectric properties of compositions in the PZ-PT-PMN ternary system with small (up to 10 mole %) substitutions of Ba or Sr for Pb. None of these papers describes the pyroelectric or low frequency dielectric properties, which would be relevant to the applications in pyroelectric infra-red detectors.
Three papers have also been published which describe the pyroelectric properties of ceramics in related compositional systems. These are by, M. Kobune, S, Fujii and K. Asada (104 J. Ceram. Soc. Japan (4) 259-263(1984)); S. W. Choi, S. J. Jang and A. Bhalla (22 J. Korean Physical Society (1) 91-96 (1989)); and M. M. Abou Sekkina and A. Tawfik (3 J. Mat. Sci. Let. 733-738 (1984)). The first of these (Kobune et al) discloses the pyroelectric properties of ceramics with the compositions: PbZr.sub.x Ti.sub.1-x O.sub.3 -0.5 Wt. % Mn0 with x=0.1 to 0.5. These compositions contain no PMN, and the crystal structure of the compositions described by this reference are tetragonal.
Choi et al discloses the pyroelectric properties of ceramics in the (1-X)Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -XPbTiO.sub.3 solid solution. Here again, the crystal structure exhibited by of all the ceramics described is Letragonal. Furthermore, the structures contain no PZ. Finally, Abou Sekkina and Tawfik discloses the pyroelectric properties of ceramics with the composition Pb.sub.1-y/2 (Zr.sub.1-(x+y) Ti.sub.x Nb.sub.y)O.sub.3. Here, the ceramics are rhombohedral but contain no Mg.
Pyroelectric ceramic materials are disclosed in Japanese patent publication nos. JP-01264962 and JP-020051426. Piezoelectric ceramics are described in European patent publication no. EP-A-0484231.